1. Field of the Invention
The present invention relates to an input buffer using components providing hysteresis to prevent the buffer output from inadvertently transitioning due to input signal noise.
2. Description of the Related Art
Noise in a circuit can cause ringing during a signal transition as illustrated in FIG. 1. If an input buffer has a fixed threshold voltage of H1, when ringing occurs the output of such an input buffer will transition on the first edge of the signal, and then transition two more times during the ringing before the second edge of the signal is reached and the signal becomes stable. A transition only on the first edge is desired.
A Schmitt trigger as shown in FIG. 2 can be used as an input buffer to prevent buffer transitioning during ringing of an input signal. To prevent transitions during ringing, the Schmitt trigger has a variable input threshold providing hysteresis. As illustrated in FIG. 3, with an input signal transitioning low to high, the Schmitt trigger has a first threshold higher than H1 prior to the Schmitt trigger output transition because the NMOS transistor 100 is on and the PMOS transistor 102 is off. However, immediately after a transition of the output of the Schmitt trigger, the buffer threshold shifts to a value lower than H1 because the NMOS transistor 100 turns off and the PMOS transistor 102 turns on. With the immediate shift to a lower threshold, signal ringing during a transition will not cause the output of the Schmitt trigger to inadvertently transition. Similarly on a high to low transition of an input signal, the Schmitt trigger has a threshold which changes from being lower than H1 to higher than H1 immediately after a transition of the Schmitt trigger output so that ringing will not cause an undesired buffer output transition.
A decrease in circuit operation speed, however, occurs when a Schmitt trigger is used, as opposed to a buffer which has a fixed threshold of H1. As illustrated in FIG. 3, on a low to high transition of an input signal, the Schmitt trigger threshold will be slightly higher than H1. The Schmitt trigger will, thus, take slightly longer to provide an output transition on a low to high input signal transition than a buffer having a threshold fixed at H1. Similarly, on a high to low transition of an input signal, the Schmitt trigger threshold will be slightly lower than H1. The Schmitt trigger will then again take slightly longer to provide an output signal transition on a high to low input signal transition than a buffer having a threshold fixed at H1.